PCM demodulators form decisions on the identity of transmitted symbols using time-invariant correlation filters. Such filters arise naturally (as "matched" filters) in such demodulators when the noise in the communications channel is Gaussian and white (WGN), signal carrier phase is unknown (non-coherent signaling) and a so-called "maximum likelihood" (ML) rule is used to form symbol decisions. ML demodulators are also used in coherent signaling where it is Possible to formulate such demodulators in terms of fixed-coefficient filters.
Practical demodulators can be derived from the maximum likelihood rule when the noise in the channel is Gaussian and white. For example, it is known that a matched filter maximizes signal-to-noise ratio and becomes a "signal replica" filter under WGN. Demodulators which incorporate such "signal replica" matched filters are herein called "ML/WGN demodulators" and are often used in the prior art to demodulate PCM signals.
ML/WGN demodulators, however, do not function efficiently when the environment includes strong interference with unknown spectra and statistics. In undefined noise conditions the matched filter is usually unknown and therefore demodulation using the replica filter produces results which are less than satisfactory. One attempt to solve this problem involves "prewhitening" the non-white noise prior to detection in the filter. This approach is disadvantageous because the prewhitening filter requires knowledge of the isolated noise statistics and because the filter introduces intersymbol interference (which can only be accounted for in subsequent detection or demodulation steps). In addition, the prewhitening filter often requires many coefficients and operates at the incoming sample frequency. Accordingly, prewhitening can thus be implemented only in an approximate sense and is costly and unreliable. Other attempts to provide predemodulation interference cancellation, such as adaptive least mean squared error (LMSE) estimation of individual signal samples, using the same estimator for all samples, also produce unacceptably high symbol error rates during detection of PCM.
There is therefore a need to provide an improved matched filter which is effective in undefined noise and which does not require prewhitening and other forms of predemodulation interference cancellation. The resulting demodulator would then be advantageous for many types of bauded signals because predemodulation interference cancellation is a general practice in strong interference and all known techniques are subject to various limitations.